Dinuclear Iron(III) and Cobalt(III) Complexes Featuring a Biradical Bridge: Their Molecular Structures and Magnetic, Spectroscopic, and Redox Properties

Abstract

Bis-bidentate ligand HL featuring two o-amidophenol noninnocent units was used to synthesize novel binuclear complexes [(L)M(•L•)M(L)](ClO), M = Fe (1) and Co (2, 3), with HL (R = CH, Cl) being the facially coordinating tetradentate coligands. Upon the synthesis, the fully reduced amidophenolate form of the ligand (L) becomes oxidized, resulting in the formation of a rare example of a biradical (•L•) bridge connecting two metal ions, as supported by X-ray crystallography. The electronic structures of the complexes have been probed by Mössbauer spectroscopy, magnetic susceptibility measurements, and electron paramagnetic resonance (EPR) spectroscopy. Species 1 contains two high-spin Fe(III) ions (S = 5/2) each coupled strongly antiferromagnetically (|J| > 150 cm; Ĥ = -2JŜŜ) with a semiquinone π-radical (S = 1/2) form of the bridging (•L•) ligand. The effective S = 2 spins of each [Fe(III)+R] monomeric unit are then weakly ferromagnetically coupled with J = +0.22 cm. Species 2 and 3 reveal very similar electronic structures: the low-spin Co(III) ion is diamagnetic, which leaves the two-spin carriers at the bridging (•L•) biradical to display an isotropic EPR signal at g = 1.995 for 2 (1.993 for 3) in solution at room temperature and in the frozen state with no hyperfine structure. The weak half-field signal at g = 3.988 for 2 (3.978 for 3) was also observed at 17 K for the spin-forbidden |ΔM| = 2 transition due to ferromagnetically coupled S = 1/2 spins (J = +47 cm) of the bridging biradical. The compounds show rich electrochemistry, displaying two (1) or four (2, 3) one-electron reversible processes. Normal and differential pulse voltammetry as well as constant potential coulometry, combined with EPR experiments, confirmed that the observed electron transfers are all localized at the bridging noninnocent (•L•) ligand.

Crossover from Antiferromagnetic to Ferromagnetic Exchange Coupling in a New Family of Bis-(μ-phenoxido)dicopper(II) Complexes: A Comprehensive Magneto–Structural Correlation by Experimental and Theoretical Study

“…We have an ongoing project on the chemistry and reactivity of transition-metal complexes involving phenol-based ligands. , Herein, we report a new family of bis-(μ-phenoxido)­dicopper­(II) complexes, 1–5 , including five structurally similar tetradentate diphenol ligands (Scheme ). Different sets of ligand substituents exert different steric pressure on the participating copper centers, which allows a large modulation of both structural parameters and magnetic properties in the series.…”

NIR-absorbing transition metal complexes with redox-active ligands

“…[1][2][3][4][5]Transition metal complexes of such ligands have attracted attention recently due to their interesting magnetic properties as well as their application in catalysis. [6][7][8][9] One example of non-innocent orthoaminophenol derivatives is 2,4-di-tert-butyl-6-((4-nitrophenyl)amino)phenol (L, see scheme 1), which is found as an in-situ generated oxidized form in a copper(I) complex [Cu(L ox )(TMPD)] + (TMPD = N,N,N',N'-tetramethylpropylenediamine). [10].…”

Unusual Dinitrogen Binding and Electron Storage in Dinuclear Iron Complexes

“…The electronic structure of 2 was then carefully elucidated with B3LYP-DFT as is commonly done by us. − Because of some ambiguity in the electronic structure determination of 2 and 5 based on experimental data, we examined different spin states, including S = 3, S = 4, S = 5, and several broken-symmetry states. The lowest energy state is a broken symmetry state showing a value ⟨ S 2 ⟩ of 12.82 (12.00 would be expected for a pure S = 3 state).…”